Earth’s Carbon Budget: The Carbon
Cycle, Sources and Sinks

1.Be
able to explain that the global system has a carbon cycle including carbon
sources and sinks.

2.Be
able to describe the difference between biosphere carbon and fossil carbon.

3.Be
able to explain what “The Big Experiment” is.

4.Be
able to explain Henry’s Law and use it to decipher temperature and CO2
concentration curves from ice core data.

5.Be
able to explain why atmospheric CO2 concentrations continue to
increase, even though land and ocean carbon sinks appear to be able to
accommodate increased amounts of CO2.

Introduction

One
thing we do in this course is look at climate change in the past and present, as
well as projections for the future.Looking to recent history, humans have been running what has been
referred to as “The Big Experiment” since
the mid-1800s.And as it turns out, this
experiment is affecting Earth’s climate.“The Big Experiment” began when we embarked on the Industrial Revolution, a pathway leading to mechanization in industry,
agriculture, and transportation.This
mechanization was accompanied by a shift away from using renewable energy
sources like wood and waterpower to using increasing amounts of fossil fuels to
run modern industry.As a result, the composition
of the atmosphere is changing and consequently affecting the rate infrared
radiation moves from the Earth back into space.In order to understand what is happening as a result of “The Big
Experiment”, you need to understand the sources, movements, and storage of
carbon.In other words, you need to
understand what the Earth’s carbon cycle and carbon budgetare, and how they work.

The Natural Carbon Cycle

The
carbon cycle includes additions of carbon to the atmosphere and simultaneous
loss of carbon from the atmosphere.Things that add carbon to the atmosphere are referred to as carbon sources, and things that remove
carbon from the atmosphere are called carbon
sinks.Table 1 lists the main carbon
sources and sinks in Earth’s natural carbon cycle.

Table 1. Some natural carbon sources and sinks.

Carbon sources

Carbon sinks

·Biological respiration (releases CO2)

·Decomposition of organic material (releases CO2)

·Swamp sediments (release CH4)

·Animal flatulence (releases CH4)

·CO2 released by the ocean

·Forest and grassland fires

·Release of carbon compounds by weathering of
exposed rocks

·Volcanoes

·Photosynthesis (removes CO2 from
the air)

·Organisms secreting CaCO3 shells or
skeletons (removes CO2)

·CO2 absorbed by the ocean

·Carbon-containing compounds being trapped in
lake and ocean sediments

The emission of
carbon into the atmosphere from natural carbon sources and the loss of carbon from
the atmosphere into natural carbon sinks has produced regular cycles in
concentrations of greenhouse gases over the past one million years or so.During recent ice ages atmospheric CO2
concentrations hovered around 180ppm, and during interglacial warm periods CO2
concentrations increased to levels of about 270ppm.These upper and lower levels have been quite
consistent because the factors affecting the carbon cycle were not changing in
any major way.

The global
carbon cycle, including natural and human-produced carbon sources and sinks, is
shown in Fig. 1.In addition to natural
sources and sinks, human have recently added anthropogenic carbon sources and
sinks to the system. New sourcesinclude
carbon emission from burning of fossil fuels, carbon release connected with
changes in land use, and human-caused fires.The main anthropogenic carbon sink is the industrial production of
materials that contain carbon.

Figure 1. The global carbon cycle.“N” refers to naturally occurring carbon pathways, “H” refers to
human-produced pathways, and “N/H” refers to pathways that occur naturally and
that are accentuated by human activities.(Image: NOAA, modified by ARH)

Fossil Carbon

The
carbon taken up by phytoplankton goes with them when they die and drift toward
the sea floor, and as they are swept into deep waters with the global conveyor.Many plankton are also are eaten by other
organisms before they reach the bottom.Of course, either they or their predators eventually die, and the carbon
in their bodies ends up on the seafloor where they are buried by sediment.Over a long period of time, the slow but
continual accumulation of carbon and sediment continues.In certain locations geologic processes
including high pressure and temperatures produced layers of sediment rich in very
high concentrations of carbon.This is
the source of oil and natural gas.

On
land, plants also carry out photosynthesis and take up CO2 that they
use to build their bodies.When plants
die the remains of their bodies add carbon to the soil.Over long time periods the slow accumulation of
this carbon continues, and eventually resulted in pockets in the Earth’s
sediments where it was subjected to intense pressure and converted into coal and natural gas.

The amount of
energy stored in these fossil fuels is
extremely large.For example, one US
gallon of gasoline has energy content of about 132 million Joules.This may not mean that much, but let’s put
this information into context with something that is easier to imagine.Elite world-class bicyclists, when riding at
full speed, can produce an average of 400 Watts of energy per hour.In other words, they produce 400 Joules per
second of work over a one-hour period.At
that rate, how many hours would one of these bicyclists have to ride before
they generated the same amount of energy that is in one gallon of
gasoline?If you do the math you will
see that they would have to go at full speed for about 92 hours to expend the
same amount of energy that is stored in one gallon of gasoline.Wow!

“The Big Experiment”

Prior to the
early 1800s humans relied mainly on renewable
energy, e.g., water power, wind power, and burning renewable fuels like
wood and animal waste (e.g., buffalo chips) to meet our energy needs.These are renewable energy sources because even
if they are temporarily depleted they can regenerate themselves, given enough
time (new trees can grow, etc.).Between
1800 and 1850, however, we embarked on a new phase of industrial and
technological development we now call the Industrial
Revolution.This is when we started
to shift away from using mainly renewable energy to using non-renewable energy – mainly fossil fuels.Since then our reliance on fossil fuels has
grown continuously, following population growth and industrial activity.

When we started
using fossil fuels we had no idea that this would affect the climate, but as it
turns out, it does.When we burn renewable
energy sources like wood or biofuels the carbon they release was already part
of the biosphere, and therefore does not change the net total amount of carbon
in the biosphere.When we burn fossil
fuels, however, we are emitting carbon into the biosphere that was removed from
the biosphere long ago and trapped in rocks.So when this carbon enters the atmosphere it produces a net increase in
total carbon in the biosphere.This is “The Big Experiment”.In other words, is the global climate affected
when we disrupt historically stable levels of carbon and other greenhouse gases
in the biosphere?

As mentioned
above, fossil fuels are extremely concentrated forms of carbon and energy.This makes them effective and attractive sources
of energy for driving our industrial world.Fossil fuels, of course, provide us with a myriad of benefits.At the same time though, the use of fossil
fuels has its downside.It is important
to understand that a very long time – millions or even hundreds of millions of
years, is required for the Earth to accumulate and produce new reserves of fossil
fuels.When we mine or burn them we are
using energy harvested from the sun by photosynthesis over eons.

Figure 2 shows
the amount of various fossil fuels produced during past geologic periods.By human life spans, it requires a LONG time
to form them.The authors of the study
who produced this graph also did calculations comparing geologic rates of
fossil fuel formation to our present rate of use of these substances for energy.The result is compelling.They state, “The amount of fuel we burn in
one year required 175,000 years to sequester.”

Figure 2. The geologic age and amount of each class of fossil fuel
formed.Most of the heavy oil was formed
between 140-23 mya (million years ago).Most of the light oil or “sweet crude” was formed between 196 and a few
million years ago.Natural gas formed at
about the same time as sweet crude, with an additional pulse of production 300-230
mya, during the Permian, when our largest coal reserves were formed. (Image: Patzek
and Pimental, 2005.)

These data tell
us some important things.First, fossil
fuels are effectively non-renewable, and once we take as much as is
technologically possible, there will be no more we can get.Some estimates state that we have a minimum
of 300 years of coal remaining, and who knows how many years of oil
exactly?Decades, certainly, but how
many?Estimates vary depending on the
sources you consult. Second, these data show how rapidly we are sending carbon
into the atmosphere, considering how long it took to form.The global carbon budget is being altered,
resulting in increasing CO2 concentrations in the atmosphere.

The Global Carbon Budget

You
are already familiar with the concept of a natural budget from our earlier
discussion on Earth’s energy budget – the balance between energy coming in and leaving
the Earth.Similarly, the atmosphere has
a carbon budget – the balance between the rate of carbon added to the atmosphere
and the rate carbon is lost rom the atmosphere.This budget describes the sources and fates of carbon as it moves
through the environment.The Global
Carbon Project calculates anthropogenic (human-produced) contributions to
Earth’s carbon budge each year.Figure 3
compares annual anthropogenic carbon emissions from 1990-2000 and 2000-2009.

Figure 3. The
amounts of carbon moved by the pathways indicated.The blue numbers indicate the amount of
carbon moved in gigatons of carbon per year between 1990-2000.The red numbers indicate the amount of carbon
moved during 2000-2009. (Image: The Global Carbon Project.)

Humans
are responsible for two significant sources of anthropogenic carbon emission
into the atmosphere that cannot be accounted for by natural processes.First is the emission of carbon from burning
fossil fuels.This also includes the
production of cement that produces a smaller, but still significant amount of
carbon emission.The second source is
carbon released into the atmosphere as a result of land-use changes.For example, when we remove vegetation,
especially forests, often the residual plant material is burned which released
carbon directly into the atmosphere.Plus,
deforestation removes a community that previously removed CO2
through photosynthesis, and we typically replace forests with a different
environment that either absorbs less CO2 or none at all.By so doing we have compromised or removed an
important carbon sink.

It
is important to note that less than half of the annual carbon emissions stay in
the atmosphere.Where does the rest
go?Terrestrial ecosystems and the ocean
absorb it.Vegetation on land surfaces
removes carbon through photosynthesis and stores it in plants and ultimately
the soil.This is most pronounced in
forests.There is also photosynthesis in
oceans, and seawater absorbs CO2 directly as well.The amount of CO2 that can be
dissolved in water is related to temperature by a well-known expression called Henry’s Law.

OK, this is
important, so pay attention!Henry’s
Law states that as water temperature increases, less CO2 is able to
stay in solution, and as water temperature decreases, more CO2 can
stay in solution.This means that cold seawater
can absorb more CO2 than warm seawater.You are probably already familiar with this
principle.Think about a soda.When you open a soda that is cold less CO2
fizzes out than when it is warm.You model
Henry’s Law if you want to.Get two cans of the same kind of soda.Cool one can in a bowl containing water and
lots of ice cubes.At the same time, warm
the other can of soda in a pan of water (don’t use a microwave, that could be a
disaster!).Oh, don’t heat the can too
much…it could explode.When you have
one cold can and one warm can, open them side-by-side and observe the
difference.OK, back to the carbon
story.

The
sizes of the two main carbon sinks – land and oceans – are not known exactly,
so the numbers shown in Fig. 3 are informed estimates.Our best data show that land and oceans together
absorb 53% of anthropogenic CO2 emitted into the atmosphere.This means that 47% of anthropogenic carbon
emissions stays in the atmosphere!This
is important, because the rate of increase of CO2 in the atmosphere
would be much greater without these sinks.This observation obviously leads to a pressing question: Will these
sinks continue to take up additional emissions of anthropogenic carbon from the
atmosphere indefinitely?

First,
consider the global distribution of the main carbon sources and sinks (Fig. 4).The main carbon sources are in the middle
latitudes, mainly in the northern hemisphere.These latitudes align with the regions of major industrial
economies.Note the carbon sinks in the
Amazon rainforest in South America, tropical Africa, and the tropical and polar
oceans.

How
much has CO2 concentration in the atmosphere changed over time?Let’s consider several time scales as we
ponder this question.First, let’s look
at the past 800,000 years.We will consider
paleoclimatology in more detail in the next reading, but all data indicate that
for the past 800,000 years the main variations in atmospheric temperatures and CO2
concentrations were associated with natural cycles of ice ages and interglacial
periods driven by Milankovitch Cycles that we have already discussed.

Scientists have
plotted temperature and CO2 concentration data from Antarctic ice
cores going back 800,000 years (Fig. 5).They discovered that global temperature changes are probably triggered by
Milankovitch Cycles, which happen first, followed by changes in
atmospheric CO2 levels.Some
people have erroneously tried to use this observation to support a hypothesis that
CO2 levels are not a significant factor in climate change.Sorry, but that is not correct.CO2 plays a large role in Earth’s
climate.So, why do CO2
levels sometimes lag behind temperature change curves?This is where knowing about the high specific
heat of water and Henry’s Law gives us a clearer picture.How?Read on.

Figure 5. Temperature, CO2 (ppm), and methane (ppb)
profiles from Antarctic ice core data.Note that the CO2 profile tends to show a slight lag time
following shifts in the temperature profile.See explanation in the reading for clarification.(Image: the Environmental Defense Council.)

When the Earth undergoes
a global temperature shift, the atmosphere and land change temperature much
more quickly than oceans do.This is
because of the high specific heat of
water.Recall that this is the
physical property of water that explains that water has to absorb a large
amount of energy to change its temperature just a little (much more than either
earth or air).And, when you consider
the huge volume of water in the oceans, it takes a MASSIVE amount of energy to
change the ocean temperature even a little bit.But, eventually sufficient energy can be absorbed, and the ocean will
warm. Recall that Henry’s Law states that cool water can hold more CO2
in solution than warm water.The lag
time between warming of the atmosphere and warming of the ocean is what you
should expect due to the high specific heat of water and Henry’s Law.That is, the ocean will heat more slowly than
land, but once it warms enough it will release additional CO2.The same thing happens in reverse when the Earth
enters an ice age.It takes a period of
time for the oceans to cool enough to absorb enough CO2 to make a
significant difference in atmospheric concentrations of that gas.This is why gradual cooling typically follows
rapid warming.

If you look again
at Fig. 5 you will see that it shows temperature, CO2, and methane
profiles, but if you look at the far right end of the graphs you will see CO2
and methane curves shooting off of the top of the chart.The concentration of CO2 reached a
high of 397ppm in May 2012 (Fig. 6; NOAA).Note the consistent range of CO2 between 180-280ppm over most
of the past 800,000 years, and the very rapid rise of CO2 in the
last century.

In 1958, the
International Geophysical Year, scientists started taking measurements of the
atmosphere, including CO2, at an observatory on the top of Mauna Loa
on the Big Island of Hawaii and at a monitoring station in the Antarctic.These are excellent sites for monitoring the
atmosphere.They are the most geographically
isolated places on Earth, and are far away from centers of industry or other
significant sources of anthropogenic CO2 emission. It is important to note the obvious trend
showing significant increases in total atmospheric CO2.Though Fig. 6 does not show this, there is an
annual cycle of decreasing and increasing CO2 levels.The decreasing part of the jagged annual cycle
of CO2 flux occurs when the northern hemisphere is entering
springtime.This is when the deciduous
forests of North America, Europe, and Asia produce leaves and grow. When they
do this they take up large amounts of CO2.Later, during the northern hemisphere fall
and winter, deciduous forests drop their leaves, these leaves decompose, and
much of the carbon stored in them is returned to the atmosphere.This produces an annual pulse in atmospheric CO2
levels.There is also a slight delay in the
atmospheric data compared to when these forests do this, because it takes some
time for the atmosphere to mix.

Other Greenhouse Gases

There
are, of course, greenhouse gases other than CO2 that affect climate.Some of these include CH4
(methane), N2O (nitrous oxide), and CFCs (chlorofluorocarbons), as
well as water vapor.The concentrations of
these gases, except water vapor, are shown in Fig. 7.Methane, CO2, and N2O levels
show increasing concentrations over the past 30 years, but many kinds of CFCs
have leveled off and some are actually in decline.You may be interested to know that CFCs and
other similar compounds are known to be the primary culprits in stratospheric
ozone depletion.Because of this,
industrial nations signed an agreement in 1986, the Montreal Protocol.The
Montreal Protocol mandated a complete ban on the production and use of
CFCs.That agreement went into effect in
1989.Some CFCs are long-lived molecules
(as long as 100 years), so it is not surprising that their levels remain high,
even more than 20 years after the Montreal Protocol went into effect.But, since CFCs are no longer being emitted
into the atmosphere in large amounts, their overall concentrations have leveled
off.The other gases, however, continue
to be emitted, and their concentrations continue to rise.

So,
what about water vapor? Water vapor is
an effective greenhouse gas, but it is different in one respect than other
kinds of greenhouse gases: water vapor cannot accumulate in the atmosphere the
way other greenhouse gases do. Water evaporates and enters the atmosphere as
water vapor.It accumulates until the
atmosphere reaches 100% humidity.The atmosphere
then drops the water vapor as precipitation, and the cycle begins again.

Anthropogenic Factors

The
direct emissions of carbon from fossil fuel use and indirect emissions due to
land use change have been tracked over time.The fossil fuel emissions have grown rapidly over the past decade (Fig.
8). The brief downturn in fossil fuel
emissions is believed to be associated with the recent economic recession.The total amount of annual carbon emissions
from land use accounts for about 10% of all anthropogenic carbon
emissions.Interestingly, land use
emissions appear to be tapering off slightly, though this does not yet
represent a significant decline (Fig. 9).

Currently,
the total annual anthropogenic emission of CO2 is just over 10 petagrams or 10 gigatons of carbon per year (a petagram is 1015 grams of
carbon, and a gigaton is 109 metric tons or 1012
kilograms).Just in case you are not
that familiar with the metric system, one kilogram is 2.2 pounds, so one metric
ton is about 2200 pounds.No matter how
you slice it, these are BIG numbers!

Next
let’s look at terrestrial carbon sinks.Global
deforestation has significantly reduced total area covered by forests, one of
the most important terrestrial carbon sinks. Figure 10 compares the best
estimates of original forest cover and the remaining forest cover on Earth.Much of this deforestation has happened in
recent years.

Now
that we have some idea about sources and sinks of CO2 on the global
scale, let’s look at regional differences of carbon emissions from burning
fossil fuels and land use changes (Fig. 11). Land use in North America is providing a small
carbon sink. Increases in eastern forest
cover over recent decades are probably responsible for this, but emissions from
industry are huge.Europe is a large
emitter, with only a small amount of emission from land-use.Asia is the biggest contributor in both
categories.Asia is cutting down its tropical
forests and increasing industrial activity, relying mainly on coal.Countries like China, India, and Indonesia
are all emerging industrial nations.South America and Africa have larger land use emissions than industrial
– this is due mainly to deforestation.China
has now surpassed the USA as the world’s largest single-nation carbon
emitter.This is related to their rapid
economic development, and abundant supplies of coal upon which they rely for their
industrial energy.

If,
however, emissions are considered on a national basis, China, the USA, India,
Russia, Japan, and Germany are at the top emitters (Fig. 12).But, when we divide total emissions by
population size, four countries stand out above the rest in per capita
emissions: the USA, Saudi Arabia, Australia, and Canada. Why is this the case?Saudi Arabia is an oil-producing nation, and
emits a lot of carbon as a by-product of that industry.Australia and Canada both have relatively
small populations, highly developed economies, and industries that require
energy to keep them running.Of the top
four, only the USA has both a high per capita and large national
emissions.This means that people in the
USA have the largest carbon footprint of any nation in the world.

Figure 12. National total and per capita carbon emissions for the
top 20 emitting nations in the world.(Image: The Global Carbon Project.)

The
concentration of CO2 in the atmosphere has continued its rapid rise
over the past few years.Figure 13 shows
the monthly CO2 concentrations from Mauna Loa in red and the updated
monthly averages in black.This graph
looks only at a 12-month average.Also
shown in Fig. 13 is the average rate of increase for each year from 2000-2009.You need to remember that these values do not
show the amount of CO2 in the atmosphere, just the rate of change in
the amount of CO2 there.

What
are the main carbon sinks, and how big are they?In other words, how much carbon are they able
to remove from Earth’s atmosphere in relation to carbon emissions?

Figure 14 shows
the annual averages of carbon flow for 2000-2009 during which an average of about
8.8 PgC was emitted per year.In 2010,
however, total anthropogenic emissions rose to 10 PgC/yr.The more worrisome thing is that it appears
that land and ocean sinks are not able to accommodate as much CO2 as
they used to, so a larger percentage of the CO2 is remaining in the
atmosphere.

The oceans and
land together currently provide a sink for only about half of the anthropogenic
carbon emitted each year.This
important, since the observed increases in atmospheric CO2
concentrations would have been much greater without these sinks.Figure 15 shows changes in annual carbon
emissions and the amount remaining in the atmosphere over the past 50
years.The amount staying in the
atmosphere shows variability from year to year.Why is this?This variability is
tied to natural variations in the amount and rate of photosynthesis and
respiration, as well as ocean temperatures.The conclusion supported by these data is that emissions have grown
rapidly, but the amount of carbon remaining in the atmosphere has increased
much more slowly, thanks to the capacity of global carbon sinks so far.

When data over
the last 50 years are examined we can see changes in the proportion of CO2
emissions staying in the atmosphere (Fig. 16).The trend line statistically fitted to the data shows that the proportion
of CO2 remaining in the atmosphere is gradually increasing, e.g., 5%
more CO2 is remained in the atmosphere in 2009 than was observed in
1960.This suggests that carbon sinks
are no longer keeping pace with carbon emissions.

Figure 16.
Fraction of CO2 remaining in the atmosphere, 1960-2009. (Image
courtesy of the PNAS.)

Figure 17 shows
graphs for land and ocean sinks over time.Negative values on that graph indicate carbon being removed from the
atmosphere.Annual variations in
temperature and the amount and timing of precipitation can have a significant affect
on photosynthesis of large ecosystems – like entire forests or grasslands – in
a given year, so the value for the land varies greatly.Because of this it is sometimes difficult to see
a readily identifiable trend for the global land sink, though a slight trend
toward absorbing more carbon may be appearing. Ocean results, on the other hand, show a clear
trend.It shows that increasing amounts
of carbon are being removed from the atmosphere each year.Yet, if the size of the combined sinks is
growing larger, why is the fraction of CO2 in the atmosphere still increasing?The short answer is that while the land and
ocean sinks are taking up larger amounts of carbon than they used to, this is
not enough to keep up with the amount of carbon being emitted into the
atmosphere.The total amount of CO2
remaining in the atmosphere is therefore rising.While this is worrisome, just imagine what
the atmosphere would be like without those sinks!